Paper-clearing: realistic market engine models

[goodboy]

Originally defined in #152 but since #462 got the position and ledger tracking part done, i'm just copying over the alt model requirement to this new task.

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More or less we want a more realistic (aka less ideal then current) set of models for the paper engine to leverage for simulating real-time market conditions.

Obviously the fanciest form of this would be some kind of real-time conditions recording system where the model would just record and then (store and) simulate what was recorded for forward tests 🏄🏼

I had a small list of libs that had some apis / types for wrapping this kinda thing:

• zipline's slippage model stuff
• zipline's internal positoin tracking system
• qtpylib just uses IB's structure verbatim as is wrapped inside ib_insync
• we currently are doing about what qtpylib does